1. Field of Invention
The present invention relates to new cristobalite-contained silica glass, a method of producing the silica glass and a silica glass jig made of the silica glass the surface of which is covered with a vapor-deposited thin film. More particularly, it relates to cristobalite-containing silica glass having .alpha.-cristobalite in the shape of a small sphere or a small, round-edged or sharp-edged, three-dimensional region dispersed in the silica glass matrix, a method of producing the .alpha.-cristobalite-contained silica glass and a silica glass jig made of the silica glass best used in a plasma etching process of semiconductor device fabrication.
2. Description of the Prior Art
Silica glass has been heretofore used in fields such as for a jig for use in semiconductor device fabrication and the like, wherein high purity, heat-resistance and chemical resistance are required, since silica glass is generally good in heat-resistance, chemical resistance and is also good in processability and cleanablity. With the recent advance in packing density of constituent elements in a chip of a very-large-scale integrated circuit, conditions under which a jig is used has been getting severer, so that it has become earnestly required that firstly any element never comes out at any extent from the jig in a semiconductor device fabrication process and thereby the jig does not contaminate a semiconductor device being fabricated, and, secondly, the jig is not subjected to thermal deformation, even at a treating temperature of higher than 1000.degree. C. In such a technical situation, crystal-containing silica glass which is excellent in not only heat resistance, but also purity has drawn attention. For example, a crystal-containing silica glass is proposed in a published document of Unexamined Patent Application No. Hei 3-45029, which silica glass is produced by sintering a mixture of crystalline quartz and amorphous quartz. Another crystal-contained silica glass is proposed in Journal of the American Ceramic Society, Vol. 42, No. 6, pp 263-270, Jun. 1, 1959, in which a crystal phase is created in the silica glass matrix by doping aluminum and the like as a crystal forming agent.
However, since the crystal-contained silica glass described in Unexamined Patent Application No. Hei 3-45029 is produced by sintering, gaps are generated among small crystals in the silica glass matrix, and thereby the mechanical strength is low and the thermal expansion coefficient is smaller than that of silica glass. When a jig made of such crystal-containing silica glass is used in a chemical vapor deposition process for a semiconductor device fabrication and is cooled in a cooling process, a thin polysilicon or silicon nitride film deposited on the surface of the jig shrinks to a great extent and develops cracks therein, since the thermal expansion coefficients thereof, which are 1.times.10.sup.-6 /.degree.C. or larger, are by far larger than that of the silica glass. Dust generated by peeling-off due to the cracks of part of the film causes the following problems. The dust floating in the atmosphere of the chemical vapor deposition process can contaminate a semiconductor device being fabricated. The cracks further advance into the body of the jig to finally break down the jig.
Because the crystal-contained silica glass proposed in the Journal of the American Ceramic Society above mentioned uses a crystal forming agent to form a crystal phase in the silica glass matrix, it has presents the problem that the crystal forming agent diffuses out impurities to thereby contaminate a semiconductor device being fabricated, when the jig is used in a temperature higher than 1000.degree. C.
Further, with the same recent advance in packing density of constituent elements in a chip and the trend for finer patterning in the chip of a very-large-scale integrated circuit, precision processing on a finer scale, on the order of submicrons, has been applied to a silicon wafer and the traditional etching treatment of the silicon wafer has to also change to the plasma etching treatment. In the plasma etching treatment, a silica glass jig has a risk of being etched itself in a special gas atmosphere and as a result, the silicon wafer has a risk to be contaminated by generated particles from the jig. In light of the above drawback, a silica glass jig the surface of which is covered with a vapor deposited thin film made of metal or ceramic thereon has been proposed, but since the thermal expansion coefficient of silica glass is usually smaller than those of the metal or ceramic, the thin film contracts to a great extent and thereby has the problem that a crack occurs in the thin film when it is used in a high temperature treatment of a silicon wafer. This problem has blocked the practical application of a thin film on the surface of a jig.